I'm not familiar with Orca but I am familiar with the use of bang in Max/MSP[0], in which it's used as a generic event trigger for attached objects. Objects receiving a bang message will execute their main method. The context feels similar here.
It seems like a general-purpose message that nodes can send to neighboring nodes, so they can be activated in response to input events, clock cycles, and so on. In other contexts you might call it a "pulse" or a "tick."
A bang in this context triggers the character one coordinate down and the one coordinate right of the bang character - which is '*' - and deletes itself after one tick.
I may have remembered some details wrong, it's been a while.
[0] https://docs.cycling74.com/max8/tutorials/basicchapter02